Energy costs can account for as much as half of a mobile operator's operating expenses, so wireless network solutions that improve energy efficiency are not only good for the environment, but also make commercial sense for operators and support sustainable, profitable business.
FIG. 1 is a schematic illustration of an example of a radio base station 100 having a number of antennas ANT1, ANT 2, ANT 3, each of which typically covers a respective sector or cell. As illustrated in FIG. 2A, a number of power amplifiers 10-1, 10-2, 10-3 are used to power the antennas 20-1, 20-2, 20-3. Normally, each antenna has its own power amplifier. For standard directional antennas, each antenna is arranged to have a radiation pattern that covers a respective angular sector, also referred to as a cell, such that all antennas together provide substantially omnidirectional radio coverage, as illustrated in FIG. 2B.
A solution for saving power involves temporarily inactivating cells into passive cells, where the associated radio base stations do not transmit any cell-defining signals for the passive cells, but only for active cells. Such an inactivation of cells not only saves power for the radio base stations but also contributes to lowering the total interference level in the radio communication network. However, the downside of this solution is of course that there is no radio coverage in the inactivated cell(s), and hence there is no support for user traffic within the corresponding geographic area. This type of solution may be thus satisfactory during periods in which there is no need for radio communication services in some of the cells.
It is known that a power amplifier and surrounding electronics consume a relatively high quiescent power, even if the output power is zero, as illustrated in FIG. 3.
It therefore makes sense to use fewer power amplifiers as the total power consumption can be reduced. A possible approach is to connect only a subset of the power amplifiers, e.g. a single power amplifier, to the existing antennas, as illustrated in FIG. 4. Unfortunately, the resulting combined radiation pattern of all the antennas will have a lot of deep nulls, i.e. very low radiated power in certain directions, so-called null-depths, as illustrated in FIGS. 5A-B. This means that this approach provides inadequate directional coverage.
It is thus desirable to reduce power consumption while ensuring adequate directional radio coverage—two seemingly conflicting requirements.
Reference [1] discloses a base station comprising an arrangement of several directional antennas, whose individual azimuthal beam patterns achieve a substantially omnidirectional coverage. The signal from a base station transceiver is split into three signals, each which is amplified and fed to a respective one of the directional antennas to provide a “pseudo-omnidirectional” pattern. The main drawback with this solution is that a number of sharp null-depths are created in the “pseudo-omnidirectional” pattern, which will cause areas of poor or no coverage. Phase shifters may be used to shift the phase of the transmitted signals in order to reduce the effect of null-depths resulting from the connection of the transceiver to more than one antenna at a time. The phase shifters may thus move the null-depths, but will generally not be able to eliminate them.
Reference [2] describes how a base station is allowed to simultaneously transmit signals in several beams of a multi-beam antenna configuration, where antenna pattern control is maintained by employing orthogonal polarization orientation for every other beam.
Reference [3] relates to an antenna arrangement configured to provide an omnidirectional radiation pattern substantially without null-depths when the radiation pattern of a number of partially overlapping beams are combined under certain conditions, including the use of different orthogonal polarizations for antennas/antenna clusters covering neighboring angular sectors.